The Bekenstein Bound sets the theoretical maximum storage density for storage media, which is equal to the entropy of a black hole of the same volume, or roughly:
$$I<=\frac{2πkRE}{ħc\ln2}$$
where I is the information expressed in number of bits contained in the quantum states in the sphere.
This works out to
$$2.5769082 × 10^{43} × M × R$$
where M is mass in kg and R is radius in metres.
A micro SD card is $0.015 m × 0.011 m × 0.001 m$, or $1.65 × 10^{-7}m^3$ in volume, and according to the info on a local office supplies shop, weighs $0.05kg$, so the theoretical maximum data it can store, using some as yet unknown quantum storage technology, possibly involving tiny black holes is $2.125949265 × 10^{35}bits$. That's $$≈ 26,574,365,812,500,000,000,000,000,000,000,000,000 Tb$$ (decimal terabytes, but SD manufacturers always use them because they make their products look bigger). Or, to express it another way, 265.75 Sextillion Terabytes.
That's a lot of information, what would you store on it? Well the thing about the Bekenstein Bound is that this is the amount of information it takes to completely describe the physical state, to the quantum level, of a volume the size and mass of say, a Micro SD Card.
That, however involves unknown technology that may never indeed exist, and if it did would probably require colossal amounts of energy. In terms of the tech that currently exists, albeit not available yet on aliexpress, a single-molecule transistor has been demonstrated, 167 picometres in diameter, "42 times smaller than the very smallest circuits currently possible". Assuming that they're somehow packed (in a cubic lattice, there are more efficient ways, but at the same time this doesn't allow space for connecting circuits etc.) into the chip with no volume used by the container, that gives you a maximum density of $$\frac{1.65 × 10^{-7}}{(1.67 × 10^{-10})^3} = 35,427,012,517,329,713,623,060$$ transistors. Supposing each transistor stores one bit, that's 4.375 ZB.
