borg usually does not let you do multiple operations in parallel. it blocks that using an exclusive lock on the repository.

there is a (potentially dangerous) hack with --bypass-lock.

for some situations, it might actually work ok without leading to problems.

practically try:

• borg create repo::archive lotsofdata plus borg mount repo
• borg mount repo plus borg create repo::archive lotsofdata
• (and other read-only operations like borg list)

Do not try multiple repo-writing ops in parallel!
Do not use a production repo for experiments, just create a temporary one!