[lolm8]

Was changing the battery in my E90... stupidly closed the boot with only the negative terminal disconnected, just want to know if its safe to jump the car from the engine terminals to try and pop the boot with just the positive terminal attached to the battery currently

[gbalthrop]

Yes, attach Jumper cables/ Booster to Jumpstart Terminals under Hood, & Boot release button at driver door will NOW work.

If you had disconnected the POSITIVE (+) Terminal as well, & that terminal was NOW contacting Chassis Ground, you would have to use another method, as that would cause "Short" when Cables attached to J/S Terminal.
George

[lordnikon]

What year is your car? Pre LCI models have a trunk "boot" key hole.

[Boston4]

You may be able to reach through the ski pass through in the rear seats and pull the release on the inside of the trunk.

[chaltau]

Originally Posted by Boston4

You may be able to reach through the ski pass through in the rear seats and pull the release on the inside of the trunk.

This is what I did when it happened to me.
Used a hanger to grab the trunk release hook and pulled on it.

I have heard the jump start method also works.

[lordnikon]

Seems like the OP is in the UK. ROW models may not have the emergency pull handle as it was not mandated by law unlike in NAR.