.. is a short form for daran an.
a short form for daran anhängen, in which the word »daran« (»at it« or »on it«) in fact is needless, since »anhängen« already means »suspend«, »append« or »attach«, i.e. »to hang (something) on (something else)« (which is the literal translation of »anhängen«), so the meaning of »daran« (»at/on it«) is already existent in the meaning of the word »anhängen« (»append«)
To say it shorter:
dranhängen = anhängen
So your sentences mean exactly the same as those two:
- Einige meiner Projekte hängen hinten an.
- Soll ich nicht genommene Pillen hinten anhängen?
Both of your sentences are incomplete, because they do not contain the element to which the project or pills are appended.
I would complete them this way:
- Einige meiner Projekte hängen an den anderen hinten dran/an.
- Soll ich nicht genommene Pillen an die genommenen hinten dranhängen/anhängen?
If you translate those sentences, you get:
- Some of my projects are appended to the others.
- Do I need to append pills, that I did not take, to the taken ones?
I don't know if those sentences are good English (English is a foreign language to me), So I try to explain the meaning of the German sentences:
There are projects that are performed in a certain sequence. You also have (other) projects, and some of them will be performed in this sequence after the other projects.
In the past you have taken pills when it was necessary, but in the last few days you have forgotten to take the pills. Now you wonder, if you should take now the pills that you have forgotten.