Infinite startup loop. firmware upgrade failed

my mycloud dl2100 was working fine for many many years. last time i checked, it did a firmware update and never started up again. its flashing blue, with fans very loud. it does this for a few minutes then powercycles and repeats the process again.

its been going on for hours and Im suspecting the last firmware update failed.

how do I recover it to a previous known working firmware, perhaps using a USB stick?

Please help. A lot of important files are on this drive.