Remove filesystem from an unpartitioned disk

An alternative to scrubbing all the data is to use the wipefs utility from the util-linux package. Run it without arguments to list the signatures found on the device:

# wipefs /dev/sdb
offset               type
----------------------------------------------------------------
0x0                  xfs   [filesystem]
                     UUID:  72f2a607-8af7-44c0-83c2-f1565cd68a1a

Then run it with -a to erase those signatures:

# wipefs -a /dev/sdb
/dev/sdb: 4 bytes were erased at offset 0x00000000 (xfs): 58 46 53 42

# wipefs /dev/sdb
#

This will not delete your data. It'll just remove the filesystem headers so that it's not mountable.

One easy (and heavy handed) way to do this would be to wipe the whole contents of the disk. The simplest way to do that would be to use dd:

$ sudo dd if=/dev/zero of=/dev/<disk> bs=1M count=500000

By the time the command ends (maybe an hour?) your whole disk will be filled with zeros.

If you're in a rush, you could kill the process with Ctl+C after a few seconds/minutes to see if you've wiped enough data for the disk to be considered as blank.