[lvlamb]

Uhh? Disk going bad?
One thing with competing for uptime and halting the machine for the sake of keeping the power bill low is that
at reboot, the filesystem is automatically checked.
(anomie said this another way).

Then,
for one, does dump use /tmp (Dunnoh)? Strange things happen on FreeBSD with full labels.
Usb? There are USB and USB. Note that USB-3 and USB-4 will add other incompatibilities. USB is a fast serial protocol. Not all file systems will use it as they use hard-drives.

For external disks, I use eSATA. Have to shutdown the system, then reboot with eSATA powered on, then mount it as any other hard-drive. With a wrapper, not in fstab !!! Don't want to have my terabytes drive on-line anytime.

Large files are better handled with Iris' ZFS or IBM's JFS: not options for the *BSDs. Am not litterate enough for XFS.

I usually use sftp to backup files to another WS.

Lastly, check the RAM as suggested.