I added 5 drives to mdadm raid array, then I noticed 3 had partitions on them already. What is going to happen?

I have a 15 drive raid6 array using MDADM on ubuntu linux 18.04.2 server. I installed 5 more drives and added them to the raid array, and it started reshaping. However, after that I noticed that three of the drives had windows partitions on them. Did I screw it up? What is going to happen in the next 30 hours of rebuild time?

Here is what it looks like:

NAME   MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda      8:0    0 465.8G  0 disk  
└─sda1   8:1    0 465.8G  0 part  /
sdb      8:16   0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdc      8:32   0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdd      8:48   0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sde      8:64   0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdf      8:80   0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdg      8:96   0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdh      8:112  1   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdi      8:128  1   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdj      8:144  1   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdk      8:160  1   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdl      8:176  1   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdm      8:192  0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdn      8:208  0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdo      8:224  0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdp      8:240  0   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdq     65:0    1   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdr     65:16   1   1.8T  0 disk  
├─md0    9:0    0  23.7T  0 raid6 /srv/patellae
└─sdr1  65:17   1   1.8T  0 part  
sds     65:32   1   1.8T  0 disk  
└─md0    9:0    0  23.7T  0 raid6 /srv/patellae
sdt     65:48   0   1.8T  0 disk  
├─md0    9:0    0  23.7T  0 raid6 /srv/patellae
└─sdt1  65:49   0   1.8T  0 part  
sdu     65:64   0   1.8T  0 disk  
├─md0    9:0    0  23.7T  0 raid6 /srv/patellae
└─sdu1  65:65   0   1.8T  0 part 

/dev/sdr, /dev/sdt, and /dev/sdu have other partitions on them. When I check them with fdisk, it looks like this:

sudo fdisk -l /dev/sdr
Disk /dev/sdr: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device     Boot Start        End    Sectors  Size Id Type
/dev/sdr1          63 3906220814 3906220752  1.8T 83 Linux

sudo fdisk -l /dev/sdt
Disk /dev/sdt: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x6745d7bd

Device     Boot Start        End    Sectors  Size Id Type
/dev/sdt1        2048 3907026943 3907024896  1.8T  7 HPFS/NTFS/exFAT

sudo fdisk -l /dev/sdu
Disk /dev/sdu: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0xf05f7577

Device     Boot Start        End    Sectors  Size Id Type
/dev/sdu1        2048 3907026943 3907024896  1.8T  7 HPFS/NTFS/exFAT

rebuild time is still going to take a while:

cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md0 : active raid6 sdu[21] sdt[20] sds[19] sdr[18] sdq[17] sdn[16] sdj[1] sdm[14] sdl[3] sdk[2] sdp[5] sdo[13] sdf[11] sdb[12] sde[6] sdg[7] sdi[4] sdd[15] sdc[10] sdh[0]
      25393984512 blocks super 1.2 level 6, 512k chunk, algorithm 2 [20/20] [UUUUUUUUUUUUUUUUUUUU]
      [=>...................]  reshape =  8.2% (160558080/1953383424) finish=1847.6min speed=16171K/sec
      bitmap: 1/15 pages [4KB], 65536KB chunk

unused devices: <none>

Two exFAT partitions on those drives! Is my data going to be corrupted? Am I going to have to remove drives one at a time and rebuild after each one?

I should have formatted them first.

Those Windows partitions will go away.

That said, IMHO your setup is not great on 2 accounts -

You really should use partitions (marked as FD) rather then raw drives for software RAID, and your RAID 6 volume is to large to provide a high degree of protection due to the number of drives and higher-then-you-probably-think likelyhood of more then 2 drives failing during a rebuild. I would be looking at RAID 10, or better ZFS or multiple RAID 1 sets and possibly LVM to lessen the risks.