Assuming the USB memory stick was formated and named disk1

In RouterOS version 6.x the following command sequence worked to rename disk1 to Disk2

/disk set 0 name=disk2

In RouterOS ver7.8 the above does not work .. name= is no longer valid

Can anyone tell me how to rename disk1 to Disk2 under routerOS v 7.8. ?

Are you sure that name is disk1 ?
Mine shows as usb1-part1.

Anyhow, instead of name, use slot (found it using F1 on command line for possible options and verified with GUI/Winbox)
/disk set 0 slot=disk2

See help page: https://help.mikrotik.com/docs/display/ROS/Disks
It's also mentioned specifically:

Note: Starting from RouterOS 7.7 disks will carry names (slots) where they are physically connected

Thank you ..

You are correct … disk1 is no longer used …the naming convention has changed … it’s like you stated.

What is really annoying about this specific CHANGE is that on many devices the change is transparent -- meaning the MOAB control file is stored successfully under disk2 -- while on some device like the hAPac2 I have lots of complaints that MOAB control file no longer get stored in disk2 ,,, they get stored somewhere but do not show up in the Files list ??? .... so I suspect corruption and will be forced to do a Netinstall if renaming /disk set X slot=disk2 does not work ... grrrr
How do I know THAT they get stored somewhere but do not show up in the Files list

The log file states the download was successful .... really???

For the hAPac2 the solution was to Reformat the USB memory stick then use
/disk/set x slot=disk2
Then Reformat top most named ID and make sure MBR is left unchecked … this worked to resolve the issue …