therein lies the rub
That is where the problem is found or located.
I tried to back up my database to a USB flash drive, but it isn't big enough. Therein lies the rub.
ADV
briskly, hard, vigorously
She rubbed the stain vigorously until it finally disappeared from the carpet.
well
She rubbed the lotion well into her dry skin before getting dressed.
gently, lightly
She lightly rubbed the stain with a soft cloth to remove it.
thoughtfully
He rubbed his eyes thoughtfully before answering the difficult question.
wearily
She wearily rubbed her sore shoulders after standing all day at work.
in, together
She rubbed her hands together nervously before walking into the interview room.
VERB + RUB
begin to
PREP
against
Her shoulder rubbed against the rough brick wall as she squeezed through the narrow doorway.
at
He rubbed at his tired eyes after working on the computer all day.
into
She rubbed sunscreen into her shoulders before going to the beach.
on
He rubbed the wet paint on the wall to spread it evenly.
with
She rubbed the stain with a damp cloth until it disappeared.