What is the difference between the -able that attaches to nouns and the -able that attaches to verbs?

There is a very good discussion of this in the OED, in the entry "-able":

Etymology: < Anglo-Norman and Old French, Middle French, French -able and its etymon classical Latin -ābilis, suffix forming adjectives, the special form taken by the suffix -bilis -ble suffix when added to verbs in -āre . Compare -ble suffix, -ible suffix, and see discussion at those entries.

The French suffix -able occurs in a great number of adjectives, both in formations within French (for examples of which see below) and in loans < Latin (e.g. capable capable adj., mirable mirable adj., potable potable adj.); both types are found from the 12th cent. The vast majority of formations within French (including all modern French formations) have always been deverbal. From verbs belonging to the first French conjugation in -er ( < Latin -āre ), the suffix was extended in French at an early date to form adjectives from verbs of all other conjugations; compare e.g. nourissable nourishable adj., périssable perishable adj. (from verbs in -ir ), mouvable movable adj. (from a verb in -oir ), défendable defendable adj., vendable vendable adj. (from verbs in -re ).

From the Middle English period onwards, the suffix is found in a large number of adjectives borrowed < French, apparently earliest in the early 13th cent. in merciable adj., and subsequently in many other loanwords and adaptations, e.g. perdurable adj., movable adj., measurable adj., agreeable adj., changeable adj., pleasable adj., etc. Less numerous are adjectives borrowed immediately < Latin, e.g. passionable adj., and adjectives borrowed from both Latin and French, e.g. profitable adj.

From these types, the suffix was soon extended to the derivation of deverbal adjectives within English, as shown clearly by hybrid formations on bases ultimately of Germanic origin (e.g. mistrowable adj., unspeakable adj., n., and adv., speakable adj., doable adj., takable adj., etc.), and probably also by formations on bases of Romance origin which are not paralleled in French (e.g. praisable adj., prayable adj.). Formations within English became very numerous during the early modern English period (compare e.g. bearable adj., breakable adj., wearable adj., etc.), and further loans < French and Latin continued unabated. This extension of the suffix to English bases resulted from the fact that the French base of many early loan adjectives featuring the suffix had also been borrowed into English, so that such adjectives were liable to reinterpretation as formed from the corresponding English verb, which then encouraged the analogical application of the suffix to English bases. Additionally, this extension was probably encouraged by early association with the adjective able adj. (to which the suffix is etymologically unrelated: see discussion at that entry), so that, e.g., eatable adj. could be reapprehended as ‘able to be eaten’; the few variant spellings of the suffix showing initial h- result from such association.

Subsequently the suffix was extended to form denominal adjectives such as saleable adj. (first half of the 16th cent.), marriageable adj. (second half of the 16th cent.), carriageable adj. (early 18th cent.), etc. This extension was probably encouraged by the numerous cases where a formally identical noun existed alongside a verb which was the base of a formation in -able (or where all three had been borrowed < French: compare changeable adj. beside change v. and change n., or debatable adj. beside debate v.1 and debate n.1). Sometimes it is difficult to tell whether an adjective was formed within English from a verb or from a formally identical noun, as e.g. in the case of rateable adj. Very occasionally formations from bases other than verbs and nouns are found, as e.g. in the probably de-adjectival formation moltenable adj. From the late 17th cent. onwards the suffix has also been added to prepositional verbs, as e.g. in come-at-able adj., uncome-at-able adj., get-at-able adj.