Will robocopy resume after being aborted?

If the folder contains a very large number of files, then it will indeed take a long time, required by the frequent accesses to the disk catalogs, for both source and target.

In addition, a USB3 disk on a USB2 port will work at the speed of USB2. The USB3 disk will adapt itself to the port and its protocol when doing the initial handshake.

You are using /MT:32 for Multi-Threaded, so telling robocopy to copy 32 files at once. This will cause 32 threads to fight for access to the disks, and will slow the copy even further, with the disk's head wasting time by going back and forth over the disk. In the case of slow USB2 copy, Multi-Threaded is not recommended.

Use Restartable mode /Z to process partially-copied files. With this option, when the copy is interrupted so some particular file is partially copied, the next execution of robocopy can pick up where it left off rather than re-copying the entire file. There are unfortunately cases where this switch will slow down the copy.

It is OK to stop Robocopy in the middle of an operation.

By default it will not re-copy the files that have already been copied.

The overhead for determining where to resume from (or essentially to check which files are the same) is very low - only seconds.

However, if you are not writing to a log (or are using /TEE to also write to a console) the /V will increase this overhead as it will take longer to write all the lines to the console.

NB: Restartable mode (as mentioned in the comments) is just for restarting individual files (e.g. if a giant file is stopped in the middle you can resume part way through instead of copying the whole file again) and is not required for resuming batch copying as asked in the question.