All my drives are single partition, 4T, 2T and 1T byte drive. The 4T always comes up as sda2 (I was expecting sda1). If I use OpenWrt 19.0 sda2 comes up and I'm able to mount no problems.
root@OpenWrt:~# cat /etc/mtab
/dev/root /rom squashfs ro,relatime 0 0
proc /proc proc rw,nosuid,nodev,noexec,noatime 0 0
sysfs /sys sysfs rw,nosuid,nodev,noexec,noatime 0 0
cgroup2 /sys/fs/cgroup cgroup2 rw,nosuid,nodev,noexec,relatime,nsdelegate 0 0
tmpfs /tmp tmpfs rw,nosuid,nodev,noatime 0 0
/dev/ubi0_1 /overlay ubifs rw,noatime,assert=read-only,ubi=0,vol=1 0 0
overlayfs:/overlay / overlay rw,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work 0 0
ubi1:syscfg /tmp/syscfg ubifs rw,relatime,assert=read-only,ubi=1,vol=0 0 0
tmpfs /dev tmpfs rw,nosuid,noexec,noatime,size=512k,mode=755 0 0
devpts /dev/pts devpts rw,nosuid,noexec,noatime,mode=600,ptmxmode=000 0 0
debugfs /sys/kernel/debug debugfs rw,noatime 0 0
none /sys/fs/bpf bpf rw,nosuid,nodev,noexec,noatime,mode=700 0 0
root@OpenWrt:~# dmesg | grep sda
[ 5.467440] sd 2:0:0:0: [sda] Very big device. Trying to use READ CAPACITY(16).
[ 5.482838] sd 2:0:0:0: [sda] 7814037168 512-byte logical blocks: (4.00 TB/3.64 TiB)
[ 5.490625] sd 2:0:0:0: [sda] 4096-byte physical blocks
[ 5.508962] sd 2:0:0:0: [sda] Write Protect is off
[ 5.513784] sd 2:0:0:0: [sda] Mode Sense: 67 00 10 08
[ 5.513922] sd 2:0:0:0: [sda] No Caching mode page found
[ 5.519257] sd 2:0:0:0: [sda] Assuming drive cache: write through
[ 21.526307] sda: sda1 sda2
[ 21.529852] sd 2:0:0:0: [sda] Attached SCSI disk
Recreating the fstab and the results are the same.