Use 2>&1 to direct stdout/stderr for a single command only among many in a BASH script

First, I assume you want the output from grep to go to stderr? 2>&1 redirects stderr to stdout. You probably want grep 1>&2.

Second, redirection will only apply to that single command. It does not persist, so start and stop the redirection is not an issue.

Third, I don't get: "success" (systemd completes fine). All you are testing is that the output from df -h mentions /mnt/Backups. Some other thing could be breaking the process.

Fourth, the status of processes in pipelines is unexpected at times. Each process gets its status recorded in an array PIPESTATUS. The status of the pipeline as a whole is complex. In particular, a pipeline with a redirection may be executed in a subshell (so the main shell retains its streams unchanged). In that case, I am ready to believe that your SUCCESS is related to the exit status of the subshell itself, rather than the last command in the pipeline.

I'm hazy about this stuff because I don't trust it enough to use it myself. My take on this would be to check text, not status, like: (untested):

MOUNT="$( df -h | grep --color=never /mnt/Backups )"
echo 1>&2 "Mount test gets ${MOUNT:-Nothing}"

if [[ "${MOUNT}" = "/mnt/Backups" ]]; then ...

This stuff is seriously hard to diagnose. I am learning using RC=$? to save the command status, echo $? to show the command status, declare -p PIPESTATUS to show the array of status, and RP="$( declare -p PIPESTATUS )" to save the array of status.

The issue is that all four of those (including the assignments to variable) are also commands, and result in the status being reset to zero. So you can never see your initial command status by more than one route. Status is ephemeral, data is forever.

My best mechanism is to get grep to count matches with grep -c, and save the returned count in a variable. You don't have to discard the filename, because if grep reads stdin it does not output a name.

As a refinement for large files, use grep -l. This exits when it sees the first match (grep -c has to keep counting to the end). grep -l outputs the filename containing the match, so the test is for "", or "(standard input)".