Altomg

Disclaimer, I have not used the app, just commenting on what has been stated here.
User says in El Cap, one must select a folder (not a disk) otherwise the app reports the disk is locked, but it worked fine in Yosemite. That indicates to me very lazy and/or lack of knowledge programming on the part of the developer, neither of which is a good sign for a utility program.
1) given it works in Yosemite, but not El Cap when you just select a drive, likely indicates that if SIP is enabled (System Integrity Protection), the program fails; that means the program is attempting to write to system folders (which under Yosemite, was still possible). Why in the world would you risk writing huge blocks of test data to sensitive system folders?
2) In any version of OS X, there are defined temporary folders that can be written too that the current user has permission to access, that the program could have used without having to ask the user. Here's three just to name a few:
/private/tmp AND /private/var/tmp AND TMPDIR environment variable.
That is what those folders are there for, for applications to write temporary files. If the developer does know that and STILL chose to write to protected system folders, shame on him. If he DIDN'T know these temp folders existed, that doesn't speak well either.
You really have to wonder about the safety of using this app. One hopes that the app cleans up after itself after writing all these test blocks.

Disclaimer, I have not used the app, just commenting on what has been stated here.
User says in El Cap, one must select a folder (not a disk) otherwise the app reports the disk is locked, but it worked fine in Yosemite. That indicates to me very lazy and/or lack of knowledge programming on the part of the developer, neither of which is a good sign for a utility program.
1) given it works in Yosemite, but not El Cap when you just select a drive, likely indicates that if SIP is enabled (System Integrity Protection), the program fails; that means the program is attempting to write to system folders (which under Yosemite, was still possible). Why in the world would you risk writing huge blocks of test data to sensitive system folders?
2) In any version of OS X, there are defined temporary folders that can be written too that the current user has permission to access, that the program could have used without having to ask the user. Here's three just to name a few:
/private/tmp AND /private/var/tmp AND TMPDIR environment variable.
That is what those folders are there for, for applications to write temporary files. If the developer does know that and STILL chose to write to protected system folders, shame on him. If he DIDN'T know these temp folders existed, that doesn't speak well either.
You really have to wonder about the safety of using this app. One hopes that the app cleans up after itself after writing all these test blocks.